Let's say we know a boy called Jonny and he goes bowling twice a week. My daughter has asked me which of the following questions are correct.
- Does Jonny go bowling?
- Doesn't Jonny go bowling?
We are wondering, why is it that the answer to both questions is "yes", although the second question starts with a negative word?
Questions, unlike statements, don't assert anything, so they can't be either true or false. Since the normal use for negation is to state that something is false, that means negation is not really needed in normal questions.
And that means that negation is available for other purposes in questions.
Language rarely wastes resources.
Negative Yes/No questions are used, like negative tag questions, to indicate the speaker's belief, or guess (the technical pragmatic term is invited inference), that the answer to the question is Yes. Affirmative Y/N questions have no such invited inference.
So, in this case, as Barrie has pointed out, the negative question indicates the speaker's belief that Jonny did in fact go bowling, and thus the question is asked as a matter of confirmation, rather than simple ignorance.
If you knew that Jonny went bowling, you wouldn’t ask the first question. If you thought he probably did, but were seeking confirmation, you’d ask the second.
